Most clinical assays for analytes in blood begin with a blood plasma separation step to prevent red blood cells from interfering with the test procedure. Red blood cells typically constitute about half of the volume of a blood sample. Unless the red blood cells are substantially removed, their presence can affect clinical assay results that are sensitive to color. Whole blood also can interfere chemically. For example, hemoglobin that is released from red blood cells can affect the performance of certain clinical assays by virtue of the iron heme group, which can act as a catalyst in some chemical reactions. The conventional manner of separating plasma from red blood cells is by centrifugation. However, recent advances in clinical test methods has led to the development of rapid test devices that can be used by untrained individuals outside of a laboratory setting. Centrifugation is not practical for use in these procedures. Consequently, an effort has been made to develop simple red blood cell separators that do not require expensive or labor intensive instrumentation. Early attempts to remove red blood cell interferences within a test strip have focused on simple assay devices that do not require a wash or separation step. An example of one such attempt is U.S. Pat. No. 4,477,575, which describes a blood filter made from glass fiber that can separate out red blood cells when whole blood is slowly trickled onto one side. This blood filter was incorporated into a clinical assay test by physically affixing it to a reagent pad that contained reagents to and which produced a color in the presence of cholesterol.
The use of membranes to separate plasma from red blood cells has been known for many years. One attempt to improve on this conventional procedure is described in U.S. Pat. No. 5,166,051. This patent describes hardening red blood cells by treating them with a salt-based crenating agent (e.g., potassium chloride). Unfortunately, such crenating agents often affect chemical reactions. Furthermore, the red blood cell separator of this patent is not readily adapted for use in a rapid and sensitive sandwich-type immunoassay test strip.
As such, a need currently exists for an improved technique of separating red blood cells from a whole blood sample in a lateral flow assay device.